High output therapeutic ultrasound transducer

ABSTRACT

A therapeutic ultrasound energy delivery system includes a probe having a vibrational transducer. A restraint is disposed about the transducer in order to exert a compressive pre-stress on the transducer. The restraint inhibits tensile failure of the vibrational transducer at high acoustic output.

TECHNICAL FIELD

The present invention is related to medical devices and systems,particularly therapeutic ultrasound systems.

BACKGROUND OF THE INVENTION

Percutaneously introduced catheters having ultrasound transducersthereon can be used to deliver localized doses of therapeutic ultrasoundenergy to various sites within a body. Such systems are ideally suitedfor treating or preventing pathological conditions such as arterialrestenosis due to intimal hyperplasia.

To achieve a high level of therapeutic effectiveness, a high amplitudeof ultrasound vibration is required. Unfortunately, the acoustic outputfrom a conventional transducer design is typically limited by theinherent properties of the piezoelectric material which forms thetransducer. Specifically, when operating typical piezoelectric ceramictransducers at high vibrational amplitudes, the ceramic tends tofracture. This transducer failure is caused by the high tensile stresseswithin the ceramic material during transducer operation, and the problemis exacerbated by the fact that although piezoelectric ceramic materialstend to have high compressive strengths, they have relatively lowtensile strengths.

SUMMARY OF THE INVENTION

The present invention provides ultrasound and other vibrationaltransducer systems comprising a vibrational transducer, typically anultrasound transducer, which can be operated at very high vibrationalamplitudes without failure. As such, the present invention providessystems to prevent the ultrasound transducer, which preferably comprisesa ceramic piezoelectric material, from breaking apart at high amplitudeoperation.

The present ultrasound transducer system is ideally suited for use in acatheter based therapeutic ultrasound energy delivery system.

In a preferred aspect, the present invention comprises a piezoelectricceramic ultrasound transducer having a restraint received therearound.The restraint is dimensioned or otherwise formed to have a structurewhich exerts a compressive pre-stress on the piezoelectric ceramictransducer element where the stress can be maintained during theoperation of the transducer. Advantageously, the compressive pre-stressprovided by the restraint operates to prevent tensile failure of theceramic transducer at high acoustic output.

In a preferred aspect, the strength of the compressive pre-stressprovided by the restraint on the transducer is approximately equal tothe tensile strength of the transducer element. As will be explained,when this occurs, the restrained transducer can provide approximatelytwice the acoustic output of a comparable un-restrained device beforetensile failure occurs.

In one exemplary aspect, the strength of the compressive pre-stressprovided by the restraint is approximately half-way between the tensilestrength and the compressive strength of the ceramic transducermaterial. As will be explained, when this occurs, the restrainedtransducer can be operated at a significantly increased output amplitudewithout failure.

In various preferred aspects, the compressive pre-stress provided by therestraint is just high enough to permit operation of the device withouttensile failure at an output amplitude determined to be safe andeffective for treating or preventing a pathological condition such asarterial restenosis due to intimal hyperplasia. In these preferredaspects, the required thickness and stiffness (as described below) ofthe restraint may be preferably kept to the minimum necessary to meetthe acoustic output requirements, thereby minimizing the size of thedevice, and minimizing the requirements of the electrical drivecircuitry, while maximizing the efficiency of the device in convertingelectric power into acoustic power.

In preferred aspects, the restraint may comprise a tensioned wire orfilament(s) which is/are wrapped around the transducer. In otheraspects, the restraint may comprise a jacket having an inner diameterwhich is initially fabricated to be slightly smaller than the outerdiameter of the transducer. The jacket is then stretched to expand to alarger diameter such that it can just be received over the transducer.The transducer is then inserted within the expanded jacket, and thejacket is then allowed to contract such that it exerts a compressivepre-stress on the transducer. Systems for fabricating the jacket from ashape memory metal such as a nickel Titanium alloy (e.g.: Nitinol™) arealso set forth.

The transducer is preferably cylindrically shaped, and may have anoptional central longitudinal bore passing therethrough, with the boredefining an inner surface of the transducer. In various aspects, theinner and outer surfaces of the transducer are covered in whole or inpart by an electrode. In alternative aspects, the opposite longitudinalends of the transducer are covered in whole or in part by an electrode.In alternate embodiments of the invention, the transducer is formed froma series of alternating annular shaped polymer and piezoelectric ceramicrings, commonly referred to as a piezoelectric stack.

In a preferred aspect of the invention, the vibrational mode of thetransducer is a relatively low frequency “breathing mode”, wherein thecircumference of the cylinder oscillates around a nominal value, and thestress within the ceramic is predominantly in the tangential direction.In this case, tensile stress from the vibration of the transducer whichmay otherwise lead to failure can be balanced by compressive pre-stressin the tangential direction applied by a wrapped jacket type restraint.

In an exemplary aspect, the transducer may be made of a PZT-8, (orPZT-4) ceramic material, but other piezoelectric ceramics,electro-strictive ceramic materials, or non-ceramic materials such aspiezoelectric crystals may be used as well.

In the aspect of the invention in which a wrapped restraint is used, thetensioned member wrapped around the transducer may be a metal wire,metal or polymeric braid, mono-filament polymer, glass fiber, or abundle of polymer, glass or carbon fibers. Wires may have circular crosssections or be formed as a ribbon or square wire. In various aspects,the wire is placed under tension when initially wrapped around theultrasound transducer so as to maintain the compressive pre-stress onthe transducer. Alternatively, the tension may be introduced after thewrapping is applied using thermal, chemical, mechanical or other type ofprocess.

Suitable materials which may be used for either of the wrapped orjacket-type restraints described herein include, but are not limited to,high tensile strength elastic material selected from the groupconsisting of steel, titanium alloys, beryllium copper alloys, nickel,titanium and other shape memory allows (e.g.: Nitinol™), and epoxyimpregnated kevlar, glass, polyester or carbon fiber. In one exemplaryembodiment of the invention, the restraint comprises a 0.001″×0.003″Beryllium Copper alloy ribbon wire having a tensile strength of 150,000psi or greater, wrapped around the transducer under 0.25 lbs of tension.

In aspects of the invention where the restraint comprises a wire orribbon wire, the restraint may comprise multiple layers of wire orribbon wrappings using thinner ribbon or smaller wire than would be usedfor a single layer of wrapped restraint. An advantage of using suchsmaller diameter wire or thinner ribbon wire would be that reducedbending stress would be experienced during the wrapping process, therebypermitting the wire or ribbon to be tensioned to a higher average stresswithout breaking. This in turn would allow a higher compressivepre-stress to be applied to the ceramic transducer element using athinner and less stiff restraint than would instead be required for asingle layer wrap of the same material.

In those aspects of the invention where the restraint comprises a wire,ribbon wire, or other fiber under tension, the wire restraint may befixed in place on the surface of the transducer by gluing, soldering orwelding, with the compressive pre-stress being maintained during theoperation of the transducer. Such fixation could be continuous or onlyat spaced apart points or regions along the contact length between therestraint and the transducer.

The use of a beryllium copper alloy wire as the restraint has numerousadvantages including its high tensile strength, (typically 150 kpsi orgreater), corrosion resistance and conductive properties. A furtheradvantage is that a beryllium copper alloy wire is easily solderable. Assuch, it may be soldered both to an outer surface of the transducer, andbetween adjacent wraps around the transducer without the need for aspecial solder tab. In addition, a beryllium copper alloy wire caneasily be soldered at temperatures below the Curie temperature of theceramic transducer material, (which is about 300° C. for PZT-8 ceramic).Typically as well, a beryllium copper alloy wire has a tensile strength/ modulus of elasticity on the order of 190 kpsi/19 Mpsi={fraction(1/100)} advantageous ration is similar to that of stainless steel whichtypically has a tensile strength /modulus of elasticity on the order of300 kpsi/30 Mpsi={fraction (1/100)}.

In the aspects of the invention where the restraint comprises a jacket,such jacket may be made from a very high strain limit material havinggood elastic properties and high tensile strength. Such a jacket couldfirst be formed and then expanded to be slipped over the transducer andthen allowed to recover, thereby radially compressing the transducer. Ifinstead fabricated from Nitinol™, the jacket can be formed and thenexpanded to be slipped over the transducer. If maintained at asufficiently low temperature, the jacket will maintain its expanded sizeas it is placed over the transducer. When the temperature is allowed torise above a critical value the jacket material will contract, therebyapplying compressive pre-stress to the transducer.

In preferred aspects, a composite polymer is applied over the outside ofthe restraint. The composite polymer is adapted to dampen longitudinalaxis vibrations, to provide an electrical insulating layer and toprovide a convenient surface to which an outer jacket of the cathetermay be attached. Suitable materials for such a composite polymerinclude, but are not limited to, materials selected from the groupconsisting of high strength adhesives such as epoxy or cyano-acrylate,and polymers such as heat-shrinkable PVDF, polyester, nylon, Pebax, PVDFor polyethylene.

The present invention also provides methods of generating and deliveringhigh levels of therapeutic ultrasound energy to a patient. Inparticular, the present invention provides methods of delivering a highoutput from a therapeutic ultrasound energy delivery system by exertinga compressive pre-stress on a piezoelectric ceramic ultrasoundtransducer with a restraint wrapped or formed to be disposed around thetransducer; and by maintaining the compressive pre-stress on thetransducer during the operation of the transducer. In various aspects,the exertion of a compressive pre-stress on the ultrasound transducer isachieved by wrapping a tensioned wire or fiber(s) around the transducer.In other aspects, exerting a compressive pre-stress on the ultrasoundtransducer is achieved by expanding a jacket to a diameter sufficient tobe received over the transducer, inserting the transducer into thejacket and allowing the jacket to contract against the outer surface ofthe transducer, or by fabricating the restraint from a shape memorymaterial such as Nitinol™ expanded to fit over the transducer and thenshrunk with heat to apply a compressive pre-stress to the transducer.

In preferred aspects, the ultrasound transducer is cylindrical in shapeand may further comprise a longitudinally extending bore therethrough.When air is disposed within this bore, the ultrasound energy emitted bythe transducer will be directed predominately radially outwards, sincevery little ultrasound energy passes from the dense ceramic transducerinto the low density air. Thus, the efficiency of the transducer can beenhanced, providing an ideal transducer system for mounting on acatheter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a cylindrical shaped ultrasoundtransducer having a wire restraint wrapped therearound.

FIG. 2 is a sectional view taken along lines 2—2 in FIG. 1.

FIG. 3 is a perspective view of a cylindrical shaped ultrasoundtransducer having a restraining jacket received thereover.

FIG. 4 is a sectional view taken along lines 4—4 in FIG. 3.

FIG. 5 is a perspective view of a transducer and restraint receivedwithin an outer coating.

FIG. 6 is an illustration of a system for wrapping a tensioned wirearound an ultrasound transducer.

FIG. 7A is a sectional view corresponding to FIG. 5, showing electrodesattached to inner and outer surfaces of the transducer, with therestraining jacket as shown in FIGS. 3 and 4.

FIG. 7B corresponds to FIG. 7A, but instead shows an electrode connectedto the outer surface of the transducer by way of a solder tab.

FIG. 7C corresponds to FIG. 5, but instead shows an electrode soldereddirectly to the restraining wire, as illustrated in FIGS. 1 and 2.

FIG. 8 illustrates a tool for expanding a jacket such that it can bereceived over the transducer.

FIG. 9 shows an alternate ultrasound transducer comprising alternatingannular piezoelectric and polymer sections.

FIG. 10 shows a stress vs. time plot for an unrestrained transducer.

FIG. 11 shows a stress vs. time plot for a restrained transducer,operating at less than optimal output.

FIG. 12 shows a stress vs. time plot for a restrained transducer,operating at optimal output.

FIG. 13 shows a plurality of the present transducers mounted to acatheter system for delivering therapeutic ultrasound to a patient.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

A problem common to therapeutic ultrasound transducers is that whenoperating an ultrasound transducer such as a piezoelectric ceramictransducer at a very high output, the transducer will tend to fracture.Accordingly, the therapeutic effectiveness of catheter based ultrasounddelivery systems have been somewhat limited since the level ofvibrational amplitude of therapeutic ultrasound energy which theirtransducers are able to emit is limited, especially over prolongedperiods of operation.

Referring to FIGS. 1 and 2, the present invention provides a system forpreventing fracture of a ultrasound transducer, (such as a ceramicultrasound transducer), when the transducer is operated at a highoutput. In a first aspect, the present invention provides a system forpreventing tensile failure in a transducer 10, by way of a wire 14 whichis wrapped tightly around transducer 10. As can be seen, transducer 10is cylindrical shaped, having an optional longitudinally extendingcentral bore 11 extending therethrough.

In various preferred embodiments, transducer 10 has a preferred outerdiameter of 0.25 to 0.02 inches, a more preferred outer diameter of0.175 to 0.03 inches, and a most preferred outer diameter of 0.100 to0.03 inches.

In various preferred embodiments, transducer 10 has a preferred innerdiameter of 0.2 to 0.01 inches, a more preferred inner diameter of 0.125to 0.015 inches, and a most preferred inner diameter of 0.05 to 0.015inches.

In various preferred embodiments, transducer 10 has a preferred lengthof 1.0 to 0.01 inches, a more preferred length of 0.750 to 0.010 inches,and a most preferred length of 0.5 to 0.01 inches.

It is to be understood, however, that the preferred dimensions set forthherein are merely exemplary and that the present invention is not solimited to the dimensions set forth herein.

In preferred aspects, the present system provides a “high output” oftherapeutic ultrasound energy, being defined herein as being greaterthan that used for diagnostic imaging. In a most preferred aspect of thepresent invention, such “high output” is equal to or greater than 1.9 MI(mechanical index). In preferred aspects, the “high output” is achievedwith an MI less than that at which cavitation damage occurs.

In preferred aspects, the present “high output” therapeutic ultrasoundsystem is operated at an exemplary frequency range of equal to, orgreater than, 500 KHz, and less than, or equal to, 3 MHz.

Preferably, wire 14 is pretensioned when initially wrapped aroundtransducer 10 such that wire 14 exerts a compressive pre-stress ontransducer 10. Wire 14 may be made of any suitable material selectedfrom the group with mechanical properties exhibited by steel, titaniumalloys, beryllium copper alloys, Nitinol™. Wire 14 may alternativelycomprise a ribbon wire, or square wire, or a multi-strand wire. Wire 14may alternatively comprise a high tensile strength elastic material suchas epoxy-impregnated polyester, kevlar, glass or carbon fiber, in eithera mono-filament or multi-filament form.

In a preferred aspect, the tensile stress in wire 14 is about 100 kpsior higher. In one exemplary aspect of the invention, the wire is a0.001″×0.0003″ Beryllium-Copper (BeCu) alloy ribbon wire under 0.3 lbs.tension, and transducer 10 is made of a PZT-8 ceramic having a 0.050″outer diameter, a 0.010″ thickness wall, and a 0.315″ length. In thisexemplary aspect, the compressive pre-stress applied to the ceramic bythe wrapped ribbon restraint is approximately 10 kpsi, which iscomparable to the reported static tensile strength of PZT-8 ceramic at11 kpsi, and significantly greater than the reported dynamic tensilestrength of 5 kpsi.

Wire 14 is adapted to provide a compressive pre-stress on transducer 10,wherein the pre-stress is preferably maintained during the operation oftransducer 10 by the resilience of the restraining wire.

In a preferred aspect, the compressive pre-stress exerted by wire 14 ontransducer 10 is approximately equal to, or greater than, the tensilestrength of the transducer. As will be explained, when the compressivepre-stress exerted on transducer 10 is approximately equal to thetensile strength of transducer 10, a doubling of output amplitude oftransducer 10 is provided. In this preferred aspect of the invention,the stiffness of wire restraint 14 (or jacket 12) needed to provide thiscompressive pre-stress is only about {fraction (1/7)} the stiffness ofthe transducer 10, therefore it does not appreciably restrain the motionof transducer 10, as follows.

The relationship between the stiffness of restraint 12 or 14 and thetransducer 10 is established by considering that the modulus ofelasticity “Y” of restraint 12 or 14 multiplied by the cross-sectionalarea of restraint 12 or 14, divided by the modulus of elasticity “Y” oftransducer 10 multiplied by the cross-sectional area of transducer 10.

For example, using the BeCu ribbon at 19 Mpsi as wire 14, and PZT-8ceramic as transducer 10, the modulus of elasticity “Y” of the BeCuribbon is approximately 1.4 times the modulus of elasticity of the PZT-8ceramic at 13 Mpsi, when the cross-sectional area of the BeCu ribbon isonly about 1/10 that of the ceramic (1 ml ribbon thickness vs. 10 ml.transducer wall thickness). The relative stiffness of the restraintversus the transducer is then:$\frac{{stiffness}_{restraint}}{{stiffness}_{transducer}} = {\frac{Y_{restraint} \cdot A_{rest}}{Y_{transducer} \cdot A_{transducer}} = {\frac{19 \cdot 1}{13 \cdot 10} \approx \frac{1}{7}}}$

In one exemplary aspect of the invention, the compressive pre-stressexerted by wire 14 on transducer 10 is approximately half-way betweenthe compressive and tensile strengths of transducer 10, therebyproviding the highest possible output without failure, (as will beexplained).

To ensure that wire 14 provides a compressive pre-stress on transducer10, it is also important to ensure that wire 14 does not simply unwrap,thereby losing its contact from the outer surface 13 of transducer 10.Accordingly, wire 14 is preferably glued or soldered against outersurface 13 of transducer 10. Alternatively, adjacent wraps of wire 14may be soldered, welded, or glued together with wire 14 being secured tothe outer surface 13 of transducer 10 by friction.

In one embodiment, wire 14 is welded, soldered, or glued to transducer10 or to adjacent wraps of wire 14 only at opposite transducer ends 15and 17. An advantage of welding wire 14 only at ends 15 and 17 is thatthis avoids relieving the stress in 10 wire 14 due to heating ormelting. As such, a circumferential weld near each of ends 15 and 17 maybe used to distribute the stress on the weld, with only a few turns ofwire 14 near ends 15 and 17 being under reduced stress, with the(unheated) center turns of wire 14 exerting the compressive pre-stresson transducer 10. Alternatively, in another embodiment, wire 14 iswelded or adhesively attached along the entire length of 15 transducer10 between ends 13 and 15.

Wire 14 may optionally be a ribbon wire, which has the advantage ofdistributing stress favorably over surface 13 of transducer 10, with theentire width of the ribbon in contact with the ceramic transducer 10,instead of just a narrow strip where a round wire would be in tangentialcontact with the cylindrical transducer surface. Furthermore, since aribbon wire provides the maximum amount of metal in a minimum profile, aribbon wire permits the maximum restraint with minimum increase in theoverall dimension of the restrained transducer. Furthermore, due to itsnarrow dimension in the radial direction, ribbon wire would experiencemuch lower bending strain during the wrapping process as compared around wire of comparable cross-sectional area per unit length. Anotheradvantage of ribbon wire is that it is resistant to stress relief duringthe welding process in which wire 14 is attached to outer surface 13,since the actual weld would only occupy a portion of the ribbon widthleaving a large remaining portion to sustain tensile stress while thewelding takes place.

In preferred aspects, wire 14 is selected from a material with anelongation at failure of greater than wire diameter/transducer radius,having the highest possible tensile strength. Alternatively, ribbon wire14 is selected from a material with elongation at failure of greaterthan wire thickness transducer radius, having the highest possibletensile strength. In either case, the lowest possible modulus is desiredso that there is a minimum of restraint exerted on transducer 10.Examples of such materials include Beryllium Copper (BeCu) alloy 172,with various tempers having tensile strengths of 100-240 kpsi andelongation of 1-10% , or various stainless steel alloys, or highstrength Titanium alloys.

In a preferred aspect, wire 14 is wrapped over itself such that amulti-layer restraint is provided. An advantage of wrapping smallerdiameter wire is that it will exhibit a lower bending stress, ascompared to a larger diameter wire wrapped around the transducer.

In one preferred aspect, opposite ends 15 and 17 of transducer 10 may beelectroded. Alternatively, in another preferred aspect, an inner surface19 and outer surface 13 may instead be electroded.

In an alternate embodiment of the invention, the restraint used to exerta compressive pre-stress on the transducer comprises a jacket receivedover the transducer. Referring to FIGS. 3 and 4, transducer 10 is shownsurrounded by a restraint jacket 12 which is slipped thereover andexerts a compressive pre-stress, similar to that exerted by wire 14, aswas described above.

Jacket 12 may preferably be formed to maintain a compressive pre-stresson transducer 10 in a number of ways. In a first aspect, jacket 12 isinitially formed with an inner diameter slightly less than the outerdiameter of transducer 10. Thereafter, jacket 12 is stretched radiallyby mechanical or thermal means to expand its inner diameter to adimension such that it can just be slipped over transducer 10, withtransducer 10 received therein as shown in FIGS. 3 and 4. After jacket12 has been slipped over transducer 10, jacket 12 will then be releasedsuch that it naturally contracts somewhat around outer surface 13 oftransducer 10. Consequently, jacket 12 exerts, and maintains, acompressive pre-stress on transducer 10 during its operation.

Jacket 12 may preferably be fabricated from a high tensile strengthelastic material, including any of the exemplary materials set forthabove with respect to wire 14. Alternatively, jacket 12 may befabricated from a shape memory metal such as Nitinol™. In this aspect ofthe invention, a change in temperature will alter the size of jacket 12such that it constricts around transducer 10 after having been receivedthereover. For example, a Nitinol™ alloy can be chosen to be Martensiticat the temperature of liquid nitrogen, and super-elastic in thetemperature range from room temperature to body temperature and slightlyabove. The Nitinol™ alloy would be austenitic at elevated temperatures.Such a material can be fabricated as a thin wall tube with innerdiameter slightly less than that of the transducer. For example, theceramic transducer could have an outer diameter of 0.050″ with a 0.010″wall thickness and a 0.315″ length. The NitinolTm tube could befabricated with an inner diameter of 0.048″ and a wall thickness of0.002″. When the Nitinol™ is cooled to liquid nitrogen temperature(˜−200° C.) the Nitinol™ becomes Martensitic and is relatively easilyexpanded to an inner diameter of 0.052″, allowing it to be slipped overthe outside of the ceramic transducer. When the Nitinol™ warms up toroom temperature, it becomes super-elastic, and it attempts to recoverto its original fabricated dimensions. The recovery is limited by theceramic, but the super-elastic alloy applies a compressive pre-stress tothe ceramic, thereby preventing premature tensile failure of theceramic.

When using either jacket 12 or wire 14 as the restraint on transducer10, such restraint will preferably have a high tensile strength so thatonly a thin layer of the restraint material will be adequate, yet alsohave to have a low stiffness such that it would not unduly restrain theceramic transducer 10.

When using either a wire restraint (FIGS. 1 and 2) or a jacket restraint(FIGS. 3 and 4), the restraint is preferably received within an outercoating 16, as shown in FIG. 5. Outer coating 16 may preferably comprisea composite polymer, which operates to dampen longitudinal vibrationsand provide an electrical insulating layer. In an exemplary aspect,outer coating 16 comprises a high strength thin wall polymer such as0.001″ thick polyester or nylon polymer, attached to jacket 12 by a highstrength adhesive, preferably having at least 500 psi shear strength.

The present invention also sets forth systems for wrapping wire 14around transducer 10 such that wire 14 remains in tension. Referring toFIG. 6, two strands of wire 14 are shown being wrapped simultaneouslyaround transducer 10 as transducer 10 is rotated in direction R. In thissystem, a pair of equal weights W1 and W2 keep wire 14 under tension aswire 14 passes over pulleys P1 and P2. Since W1 and W2 are equal, thewires 14 will not produce any net bending stress on the transducer 10which could cause it to break during the manufacturing process.Alternatively, weight W2, pulleys P1 and P2 and one wire 14 may beeliminated to simplify the wrapping fixture. In this case, thetransducer 10 must be strong enough to resist the bending stress createdby the tensioned wrapping wire 14.

Longitudinally extending bore 11, as seen in FIGS. 1 to 5, maypreferably be air filled. Advantages of an air-filled bore include thefact that ultrasound energy can not be transmitted thereacross. Instead,all of the ultrasound energy emitted by transducer 10 willadvantageously be reflected off of inner surface 19, and directedradially outwardly, thereby increasing the therapeutic effectiveness oftransducer 10. Another advantage of air-filled bore 11 is that it can beused for passage of a guidewire therethrough.

FIG. 7A shows an embodiment of the present invention in which jacket 12is made of Nitinol™, with an electrical lead 22 passing under outercovering 16 and through a hole 9 passing through jacket 12 such that anelectrical lead 22 may be attached to electroded outer surface 13 oftransducer 10. Similarly, an electrical lead 24 is attached to the innersurface 19 of transducer 10 as shown. FIG. 7B shows electrical lead 22connected to electroded outer surface 13 by way of a solder tab 18. FIG.7C shows electrical lead 22 soldered directly to electrically conductivewire 14, which is in direct contact with electroded outer surface 13 oftransducer 10.

In a preferred aspect, wire 14 is soldered at ends 15 and 17 to preventunwrapping from transducer 10. The outer electrode connection may bemade by soldering directly to wire 14. As such, transducer 10 can bewrapped all the way from end-to-end with no unwrapped segment requiredfor lead attachment.

FIG. 8 illustrates a tool for expanding jacket 12 such that it can bereceived over transducer 10. The tool comprises a split mandrel 20 and atapered conical wedge 21. Conical wedge 21 is inserted into a borepassing through split mandrel 20 such that jacket 12 can be expanded. Ina preferred aspect, jacket 12 is made of Nitinol™, and the insertion ofwedge 21 into mandrel 20 is preferably done at a cool temperature suchthat when Nitinol™ jacket 12 returns to a warmer temperature, it willtend to retract radially inwards. In an exemplary aspect, Nitinol™jacket 12 will have a thickness of approximately 0.002″, offering animproved compromise in terms of strength and low restraint.

In preferred aspects, transducer 10 will be operated at a lowtemperature rise. Such low temperature rise can be achieved bymaintaining a low duty cycle, or alternatively by providing a coolingflow such as a saline infusion over transducer 10 during its operation.Preferably, a temperature rise of less than 5° C. will be achieved.Preferably, the fluid could be introduced through an annular spacebetween transducer 10 and a polyimide guidewire sleeve. Temperaturemonitoring by a catheter mounted thermistor or thermocouple can also beused.

Referring to FIG. 9, an alternate transducer system is provided withtransducer 30 comprising alternating annular sections of PZT ceramic 32and polymer 34. Transducer 30 is ideally suited to avoiding longitudinalfailure. In accordance with the present invention, transducer 30 may besubstituted for transducer 10 in any of the above described embodimentsof the present invention. For example, transducer 30 is preferablyrestrained by a wire 14 wrapped therearound, or a jacket 12 slippedthereover, the restraint used in turn being received within outercovering 16, as described.

As stated above, the strength of the compressive pre-stress provided bywire 14 or jacket 12 on transducer 10 is at least approximately equal tothe tensile strength of the transducer material and more preferably,approximately ½ way between the tensile and compressive strengths of thematerial. This is explained as follows.

Referring to FIG. 10, a stress vs. time plot for an unrestrainedtransducer is shown. Acoustic vibrations in the transducer arecharacterized by oscillation in the stress. In a conventionaltransducer, without a pre-stress, the stress oscillates around zero,alternating between compressive (positive) stress and tensile (negative)stress.

Since piezo-electric ceramic materials typically have much highercompressive strengths compared to their tensile strengths, compressivepre-stress permits higher acoustic amplitude without subjecting theceramic to tensile stress beyond its limit. Specifically, the tensilestrength of the transducer material is shown by line 50 and thecompressive strength of the transducer material is shown by line 52. (Ascan be seen, line 50 is closer to zero than line 52, thus indicatingthat the transducer is more likely to fail in tension than incompression). If the stress during one of the cycles of oscillationexceeds the tensile strength of the ceramic, then the transducer willfracture. Accordingly, when operating an unrestrained transducer, themaximum tensile stresses will equal the maximum compressive stresses.Accordingly, the maximum peak-to-peak amplitude of the oscillations inthe stress (i.e.: the difference between lines 50 and 70) will be doublethe tensile strength (i.e.: the difference between zero and line 50) ofthe transducer material.5

FIG. 11 shows a stress vs. time plot for a transducer with a restraintwrapped therearound. In this aspect of the invention, the compressivepre-stress (labeled as distance “B”), (ie: the difference between zeroand line 54) is equal to the tensile strength (labeled as distance “A”),(i.e.: the difference between zero and line 50) of the transducermaterial. Thus, line 54 is at the same level as line 70. As can be seen,the application of such a compressive pre-stress to the transducerresults in a doubling of the maximum peak-to-peak amplitude ofoscillation in the stress relative to that of a comparable unrestrainedtransducer, (i.e.: the difference between line 56 and zero is twice thedifference between line 54 and zero).

FIG. 12 shows a stress vs. time plot for a transducer with a restraintwrapped therearound, operating at optimal output. In this aspect of theinvention, the compressive pre-stress applied by the restraint (line 58)is set to be ½ way between the tensile strength (line 50) and thecompressive strength (line 52) of the transducer material. As can beseen, the application of such a compressive pre-stress on the transducereffectively maximizes the peak-to-peak amplitude of the oscillation inthe stress to a level corresponding to the difference betweencompressive strength (line 52) and the tensile strength (line 50).

Accordingly, in preferred aspects of the invention, the compressivepre-stress applied to the transducer by the restraint is at least equalto, and preferably greater than, the tensile strength of the transducer.More preferably, the compressive pre-stress applied to the transducer bythe restraint is of an amplitude greater than the tensile strength ofthe material and not exceeding ½ way between the tensile and compressivestrengths of the material. In an optimal aspect of the invention, thecompressive pre-stress is equal to a level ½ way between the tensile andcompressive strengths of the material.

In another preferred aspect of the invention, the compressive pre-stressapplied to the transducer is sufficient to permit reliable operation atthe desired acoustic output amplitude, without permitting tensilefailure of the ceramic and without requiring an unnecessarily stiff orbulky restraint.

As such, FIGS. 11 and 12 provide illustrations of how compressivepre-stress permits higher amplitude acoustic vibrations without stressexceeding the tensile strength limit of the ceramic compressive strengthof ceramic.

Lastly, FIG. 13 is an illustration of a plurality of the presentcylindrically shaped high output ultrasound transducers 10, with wrappedwire restraint 14 thereover, as previously described herein, mountedalong a flexible catheter 60 with spacers 62 disposed therebetween.Spacers 62 may be formed from a flexible polymer material so as topermit catheter 60 to flex between the rigid transducer (10) segments.Outer covering 16 may preferably be formed from a flexible polymer whichbonds to jacket 12, and provides a smooth outer surface for catheter 16.A plurality of optional bushings 64 are disposed between transducers 10and spacers 62, forming an air gap 65 adjacent the inner surface 66defining lumen 67 through which guide wire 68 passes, as shown. In apreferred aspect, the guidewire lumen 67 is lubricious and flexible andcontains guidewire 68 and has a fluid (such as saline) passingtherethrough to provide cooling for transducers 10. Air gap 65 operatesto direct the ultrasound energy emitted by transducers 10 radiallyoutwardly, by inhibiting radially inward ultrasound emissions. Apreferred material for guidewire lumen 67 is high density polyethylene.

What is claimed is:
 1. A therapeutic ultrasound energy delivery system,comprising: a probe for contact on or in a target location in apatient's body; a vibrational transducer; and a restraint disposedaround the transducer, wherein the restraint exerts a compressivepre-stress on the transducer.
 2. The therapeutic ultrasound energydelivery system of claim 1, wherein the compressive pre-stress on thetransducer is at least equal to the tensile strength of the transducer,wherein the restraint inhibits tensile failure of the vibrationaltransducer at high acoustic output.
 3. The therapeutic ultrasound energydelivery system of claim 1, wherein the value of the compressivepre-stress on the transducer is greater than value of the tensilestrength of the transducer and less than one-half of the sum of thevalues of the compressive strength and the tensile strength of thetransducer.
 4. The therapeutic ultrasound energy delivery system ofclaim 1, wherein the value of the compressive pre-stress on thetransducer is approximately equal to one-half of the sum of the valuesat the compressive strength and the tensile strength of the transducer.5. The therapeutic ultrasound energy delivery system of claim 1, whereinthe transducer is cylindrically shaped.
 6. The therapeutic ultrasoundenergy delivery system of claim 1, wherein the restraint maintains thecompressive pre-stress during operation of the transducer.
 7. The systemof claim 1, wherein the restraint comprises: a jacket slipped over thetransducer.
 8. The system of claim 7, wherein the jacket is stretched toan expanded diameter to be received over the transducer.
 9. The systemof claim 7, wherein the jacket is formed of a shape memory metal. 10.The system of claim 1, wherein the restraint comprises: a wire wrappedaround the transducer.
 11. The system of claim 10, wherein the wire is amonofilament or multifilament polymer.
 12. The system of claim 10,wherein the wire is wrapped around the transducer under tension.
 13. Thesystem of claim 10, wherein the wire is a ribbon wire.
 14. The system ofclaim 10, wherein the wire is soldered to the ultrasound transducer. 15.The system of claim 10, wherein the wire is welded to the ultrasoundtransducer.
 16. The system of claim 15, wherein the wire is welded toopposite longitudinal ends of the transducer.
 17. The system of claim15, wherein the wire is welded to the transducer along the length of thetransducer.
 18. The system of claim 10, wherein the wire is glued to theultrasound transducer.
 19. The system of claim 1, wherein the transduceris selected from the group consisting of a piezoelectric ceramic, anelectrostrictive ceramic and a piezoelectric crystal.
 20. The system ofclaim 1, wherein the transducer is made of PZT-8 or PZT-4 ceramicmaterial.
 21. The system of claim 1, wherein the restraint is made of ahigh tensile strength elastic material selected from the groupconsisting of steel, titanium alloys, beryllium copper alloys,nickel-titanium alloys, and epoxy impregnated kevlar, polyester orcarbon fiber.
 22. The system of claim 1, further comprising: a compositepolymer outer covering disposed around the outside of the restraint. 23.The system of claim 22, wherein the composite polymer outer coveringcomprises a combination of materials selected from the group consistingof a high strength epoxy, cyano-acrylate, polyester, PVDF, Pebax, nylonor polyethylene.
 24. The system of claim 1, wherein the ultrasoundtransducer has a central air-filled bore passing longitudinallytherethrough.
 25. The system of claim 24, further comprising: a firstelectrode disposed on the outer surface of the transducer; and secondelectrode is disposed on the inner surface of the transducer.
 26. Thesystem of claim 1, further comprising: first and second electrodesrespectively disposed on opposite longitudinal ends of the transducer.27. The system of claim 1, wherein the transducer comprises a series ofalternating annular shaped polymer and piezoelectric ceramic ringshaving inner and outer surfaces.
 28. The system of claim 27, farthercomprising first and second electrodes attached to inner and outersurfaces of the transducer.
 29. The system of claim 27, furthercomprising electrodes attached to opposite longitudinal ends of each ofthe piezoelectric ceramic rings.
 30. A method for delivering vibrationalenergy to a patient, comprising: introducing a vibrational transducer tothe patient; energizing the transducer to deliver vibrational energy tothe patient, wherein the transducer is constrained by a restraint whichprovides a compressive pre-stress which permits the transducer tooperate at on acoustic output having a value which is greater than thevalue of acoustic output that would have been achievable without therestraint.
 31. The method of claim 30, wherein the value of thecompressive pre-stress on the transducer is at least equal to the valueof the tensile strength of the transducer.
 32. The method of claim 30,wherein the value of the compressive pre-stress on the transducer isgreater than the value of the tensile strength of the transducer andless than one-half of the sum of the values of the compressive strengthand tensile strength of the transducer.
 33. The method of claim 30,wherein the value of the compressive pre-stress on the transducer isapproximately equal to one-half of the sum of the values of between thecompressive strength and the tensile strength of the transducer.
 34. Themethod of claim 30, wherein exerting the compressive pre-stresscomprises wrapping a tensioned wire around the transducer.
 35. Themethod of claim 30, wherein exerting a compressive pre-stress on theultrasound transducer comprising stretching a jacket to a diametersufficient to be received over the transducer and inserting thetransducer into the jacket.
 36. The method of claim 30, furthercomprising: providing a composite polymer outer cover wrapped around therestraint.
 37. The method of claim 30, further comprising: operating thetransducer with first and second electrodes respectively disposed onopposite longitudinal ends of the transducer.
 38. The method of claim30, wherein the transducer has a central air-filled bore passinglongitudinally therethrough defining an inner surface and outer surfaceof the transducer, further comprising: operating the transducer withfirst and second electrodes respectively disposed on the inner and outersurfaces of the transducer.
 39. The method of claim 30, furthercomprising: cooling an inner bore in the transducer with a fluid flow.40. The method of claim 39, wherein the fluid flow is a saline infusion.41. The method of claim 30, wherein the vibrational transducer isoperated at a Mechanical Index (MI) of at least 1.9.
 42. The method ofclaim 30, wherein the vibrational transducer is operated at a frequencyof at least 500 KHz.
 43. The method of claim 30, wherein the vibrationaltransducer is operated at a frequency not exceeding 3 MHz.
 44. Atherapeutic ultrasound catheter system comprising: a catheter; aplurality of vibrational transducers disposed along the length of thecatheter; a restraint disposed around each transducer, wherein eachrestraint exerts a compressive pre-stress on one of the transducer,wherein the restraint inhibits tensile failure of the vibrationaltransducer at high acoustic output; and a plurality of spacers disposedbetween each of the successive vibrational transducers.
 45. A method oftreating arterial restenosis, comprising: inserting the catheter systemof claim 44 into a patient's artery; and emitting ultrasound energy fromthe plurality of vibrational transducers.